1. Field of the Invention
The present invention relates to a fuel cell hydrogen recovery system which utilizes a hydrogen absorbing alloy adapted for recovering hydrogen (purged hydrogen) which is discharged from a fuel cell.
2. Description of the Related Art
A proton exchange membrane fuel cell (PEFC) has an MEA (membrane-electrode assembly) in which a solid polymer membrane (an electrolyte membrane) is held between an anode and a cathode so that electricity can be generated even at normal temperatures, and therefore, the proton exchange membrane fuel cell can be expected to be used for various applications including an application to a power supply for electric vehicles.
Raised as a method for supplying hydrogen for use as a fuel for the proton exchange membrane fuel cell are a method (of pure hydrogen type) in which hydrogen in the form of pure hydrogen is supplied to a fuel cell from a liquid hydrogen tank which stores therein liquid hydrogen, a high pressure hydrogen tank which stores therein hydrogen under high pressure or a hydrogen absorbing tank which stores therein hydrogen supplied to a hydrogen storage material such as a hydrogen absorbing alloy which absorbs and retains hydrogen in the interior thereof and a method (of reforming type) in which hydrogen rich gas generated from hydrocarbon such as a methanol aqueous solution through a steam reforming process is refined and hydrogen rich gas so refined is then supplied to a fuel cell.
Among automobiles provided with fuel cells to which hydrogen is supplied using the method of pure hydrogen type (hereinafter, referred to as fuel cell vehicles), in a fuel cell vehicle of a highly pressurized hydrogen storage type which has a high pressure hydrogen tank, hydrogen stored in the high pressure hydrogen tank is normally regulated with respect to its pressure in a few steps down to a predetermined pressure before it is supplied to the fuel cell in order to prevent a damage to a fuel cell stack.
In addition, in the fuel cell vehicle constructed as described above, except for a case where the amount of hydrogen stored in the high pressure hydrogen tank is nearly zero, the pressure of hydrogen is reduced from a high pressure condition via a medium pressure condition to a low pressure condition by regulators along a hydrogen supply line which supplies hydrogen from the high pressure hydrogen tank to the fuel cell, and the hydrogen so regulated is further regulated with respect to its pressure down to a lower pressure condition suitable for operation of the fuel cell. A fuel cell system of highly pressurized hydrogen storage type which is adapted for domestic use also has substantially the same construction.
Furthermore, in the proton exchange membrane fuel cell, when a gas containing hydrogen (a fuel gas) is supplied to an anode, whereas an oxidizing agent gas (air) is supplied to a cathode, electrons discharged from the hydrogen gas at the anode to thereby produce protons (ionized hydrogen). Protons produced at the anode pass through the proton exchange membrane (the electrolyte membrane) to reach the cathode together with water, where the protons are oxidized by oxygen in the air to thereby produce water.
Note that since the proton exchange membrane (the electrolyte membrane) always needs to be humidified in order to facilitate the movement of protons through the membrane, the gas containing hydrogen (the fuel gas) and the oxidizing agent gas (air) are humidified before they are supplied to the fuel cell.
While the proton exchange membrane fuel cell is kept at a high temperature by virtue of heat generated through a chemical reaction (thermal reaction) between hydrogen and oxygen in the air when electricity is generated, an electricity generating location of the fuel cell which is closed to an outer peripheral part of the fuel cell is cooled more than a central part thereof. As a result, water in the humidified fuel gas causes a moisture condensation or water produced at the time of generating electricity causes an aggregation along a flow path of hydrogen supplied to the proton exchange membrane fuel cell, and an occlusion of the flow path is caused by water which has so condensed or aggregated to thereby reduce the area of the electricity generating location, leading to a risk that the function of the fuel cell is deteriorated.
In addition, at a cold time such as in winter, there occurs a case where the temperature of the fuel cell lowers below 0° C. while the operation of the fuel cell is stopped. As this occurs, reaction-generated water remaining in the fuel cell gets frozen and then blocks the hydrogen flow path, whereby the fuel cell is not allowed to generate electricity, or such blocking occurs partially along the hydrogen flow path, whereby only portions of the fuel cell is allowed to generate electricity, and the electricity generating portions of the fuel cell are excessively heated relative to the total power output of the fuel cell, leading to a risk of inducing a damage to the electrolyte membrane.
As has been described heretofore, in the event that the hydrogen flow path gets blocked due to water so frozen within the fuel cell, since an unfavorable thing such as a decrease in electricity generating efficiency or power output occurs, there exists, for example, an operating method for feeding an excessive amount of hydrogen which exceeds an amount of hydrogen required for generating electricity to the anode side so as to purge the fuel cell of the excess water which is blocking the flow path together with the excess hydrogen so fed.
After having been used as the means for eliminating the blockage of the hydrogen flow path, hydrogen so discharged from the fuel cell (hereinafter, referred to as purged hydrogen) is desirably recovered for reuse as fuel. To this end, making use of, for example, hydrogen absorbing alloys (hereinafter, referred to as MH) which can store a relatively large amount of hydrogen has been studied as a method for recovering and reusing the purged hydrogen.
There is a technique such as disclosed in, for example, JP-A-5-225996(Pages 1 to 5, FIG. 2) as a fuel cell hydrogen recovery system which utilizes the MH.
As shown in FIG. 7, this fuel cell hydrogen recovery system includes a hydrogen absorbing/discharge means 100 for absorbing and discharging hydrogen substantially continuously under a desired pressure, and this hydrogen absorbing/discharge means 100 includes, in turn, two MH's 101, 102 which are adapted for absorbing or discharging hydrogen, an absorbing changeover means 103 and a discharge changeover means 104 which are adapted for implementing a changeover between a destination to which hydrogen is absorbed and an origin from which hydrogen is discharged as required and mixing means 105, 106 which are adapted for mixing heating water with cooling water at a predetermined ratio which allows the respective MH's 101, 102 to be set to a temperature corresponding to the desired pressure when the heating water and cooling water so mixed together is supplied to the MH's 101, 102.
According to this construction, there can be provided a fuel cell electricity generation system which can enable the stable and economic supply of hydrogen to the fuel cell and which can make the fuel cell smaller in size.
With the fuel cell hydrogen recovery system, however, in order to heat and cool the MH's 101, 102, there are provided the mixing means 105, 106 for mixing heating water with cooling water at the predetermined ratio which allows the respective MH's 101, 102 to be set to the temperature corresponding to the desired pressure, and moreover, special equipment such as a heater or a burner needs to be provided additionally as a heating device for causing recovered hydrogen to be discharged from the MH's.
In addition, since piping is required for supplying and discharging heating water and cooling water, the amount of heat that is dissipated to the outside of the system from the piping so provided becomes large, and this has been not preferable from the viewpoint of energy efficiency.